We have shown that the induction of photoreactivatable damage in E. coli AB2480 by high-energy X-ray is due to the associated Cerenkov emission. By use of various experimental techniques we have been able to maximize the yield of photoreactivatable damage as measured by cell survival with and without photoreactivation. We now propose to measure the yield of thymine dimers in DNA of X-irradiated E. coli AB2480 under similar experimental conditions to our cell survival studies. If thymine dimers are the sole photoreactivatable lesions, then survival data and thymine dimer yield measurements should correlate.